1. Field of the Invention
This invention relates to an active matrix liquid crystal display device comprising first and second spaced supporting plates, a row and column array of picture elements each defined by a respective one of a set of electrodes carried on the first plate and an opposing electrode carried on the second plate with liquid crystal material therebetween, and a set of row address conductors on the first plate to which selection signals are applied by a driver circuit, each row of the picture element electrodes on the first plate being connected to an associated one of the set of row address conductors through a respective switching device on the first plate, and each switching device having an associated light shield carried on the first plate.
2. Description of the Related Art
Active matrix addressed liquid crystal display devices of the above kind and employing thin film transistors, (TFTs), as switching devices for the picture elements are generally well known. In these display devices, the gates of the TFTs are connected to the row address conductors to which gating signals are applied in sequence and the source terminals of the TFTs are connected to a set of column address conductors also on the first plate, to which data signals are applied. A single electrode common to all picture elements is provided on the second plate. Such display devices can be used to display monochrome or color video pictures, for example TV pictures. The photoconductive properties of the semiconductor material used in the addressing TFTs, particularly for example amorphous silicon or polysilicon materials, can influence significantly the off resistance of the transistor which results in poor display quality if this resistance is not adequately high. The display degradation caused by photocurrents can be especially severe where the display device is used as part of a projection display system in view of the high light intensities.
One common approach to satisfying the need for light shielding of the switching devices has been to provide a light shield for each switching device which extends over the structure of the switching device on the first plate. Shields comprising pads of light absorbing insulating material have been used but these forms of light shields are not always sufficiently absorbent and produce heat as a result of the absorption of light. It is known also to provide a light shield in the form of an isolated pad of light reflecting metal overlying the semiconductor channel region of the TFT and separated therefrom by an intervening insulator layer. Discrete metal pad light shields are much better at shielding the channel region and do not generate heat, However, their use has been severely limited as they have suffered from the disadvantage of introducing undesirable parasitic capacitance effects, for example between the source and drain terminals or the source or drain and gate terminals of a TFT depending on the orientation of its gate with respect to the shield. Also they can float to such a potential that they begin to act like a gate terminal of the TFT.
In the English-language abstract of JP-A-63-276031, it is proposed that a metal pad light shield for a TFT is connected electrically to the column conductor (video signal line) associated with and physically adjacent the TFT concerned so as to avoid problems caused by the light shield becoming electrified during the manufacturing process. While those problems may be avoided, the disadvantage of deleterious capacitive coupling effects during operation remains, especially in the case where the light shield overlies source and drain regions as well as the channel region of the TFT as would be preferable.